Substring is causing a String index out of range error

Question

I am writing a program that reverses 2 4 digit numbers, and I wrote a method that will do it. There are no errors in the project build, but I get "String index out of range: -3" when I try to run it. I am fairly new to programming and I have no idea what I did wrong.

Here's the code:

    public static void main(String[] args)
    {
        int num1 = 1;
        int num2 = 1;

        Scanner console = new Scanner(System.in);

        while (num1 <1000 || num1 > 9999)
        {
            System.out.println("Please enter a positive 4 digit number");
            num1 = console.nextInt();
        }

        String numString1 = Integer.toString(num1);

        while (num2 <1000 || num2 > 9999)
        {
            System.out.println("Please enter another positive 4 digit number");
            num2 = console.nextInt();
        }

        String numString2 = Integer.toString(num2);

        int numReverse1 = stringReverse(numString1);

        int numReverse2 = stringReverse(numString2);

        System.out.println(numReverse1 + numReverse2);

        System.out.println("The product of your 2 reversed numbers is: " + (numReverse1 * numReverse2));

    }

    public static int stringReverse (String numString)
    {
        String c1c2c3 = numString.substring(4);
        String c2c3c4 = numString.substring(1);

        String c1c2 = c1c2c3.substring(3);
        String c3c4 = c2c3c4.substring(1);

        String c1 = c1c2.substring(2);
        String c2 = c1c2.substring(1);
        String c3 = c3c4.substring(2);
        String c4 = c3c4.substring(1);

        String numStringReverse = c4 + c3 + c2 + c1;

        int reversedString = Integer.parseInt(numStringReverse);

        return reversedString;
    }
}

Fausto Carvalho Marques Silva · Accepted Answer · 2020-05-24 17:51:49Z

1

In the line:

String c1c2c3 = numString.substring(4);
String c1c2 = c1c2c3.substring(3);

You are trying to access a position that does not exists. Arrays in Java are started by 0 til " - 1".
Start index is inclusive, end index is exclusive in substring method.

A correct version for stringReverse` would be this:

public static int stringReverse (String numString)
{
    String c1c2c3 = numString.substring(0,3);
    String c2c3c4 = numString.substring(1,4);

    String c1c2 = c1c2c3.substring(0,2);
    String c3c4 = c2c3c4.substring(1,3);

    String c1 = c1c2.substring(0,1);
    String c2 = c1c2.substring(1,2);
    String c3 = c3c4.substring(0,1);
    String c4 = c3c4.substring(1,2);

    String numStringReverse = c4 + c3 + c2 + c1;

    int reversedString = Integer.parseInt(numStringReverse);

    return reversedString;
}

Also, try reading this answer.

edited May 24, 2020 at 17:51

answered Oct 25, 2017 at 15:12

Fausto Carvalho Marques Silva

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Comments

gil.fernandes · Accepted Answer · 2017-10-25 15:26:03Z

0

There are perhaps more elegant ways to reverse a string in Java. If you want an easy alternative in Java look at the class java.lang.StringBuilder.

Check the documentation and look for the reverse method.

answered Oct 25, 2017 at 15:26

gil.fernandes

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